Probable free radical effects on rat liver nuclei during early hepatocarcinogenesis with a choline-devoid low methionine diet

Fischer-344 rats fed a choline-devoid diet show lipid peroxidation in the liver nuclei, beginning at 1 day, reaching a peak at 3 days, and subsequently declining by 35 days. Lipid peroxidation in the mitochondria was seen first at 3 days, increased to a maximum at 28 days, and decreased after 35 days to undetectable values at 49 days. Lipid peroxidation was found in both nuclear and mitochondrial fractions both before and after stripping of their outer membranes. No microsomal lipid peroxidation could be detected at any time up to 63 days. The animals fed the same diet supplemented with choline showed no lipid peroxidation in any liver fraction. Animals given CCl4 showed the expected lipid peroxidation in the microsomes but not in the nuclear fraction. The administration of the free radical trapping agent, N-tert-butyl-alpha-phenylnitrone, prevented completely or almost so, microsomal lipid peroxidation induced by CCl4 and nuclear lipid peroxidation in the animals fed the choline-devoid, low methionine diet. The genesis of free radicals in the livers of rats fed a choline-devoid diet is considered as a likely hypothesis for the observed lipid peroxidation. The lipid peroxidation in turn is considered to be closely related to the induction of liver cell death and to the production of alterations in DNA. The DNA alterations coupled with regenerative liver cell proliferation suggest an attractive hypothesis for the initiation of hepatocarcinogenesis in rats fed a choline-devoid diet.

Alternative methods of selecting rat hepatocellular nodules resistant to 2-acetylaminofluorene

Dietary 2-acetylaminofluorene (2-AAF) coupled with a stimulus for cell proliferation such as a 2/3 partial hepatectomy (PH) or a necrotizing dose of carbon tetrachloride is frequently employed to generate nodules of resistant ("initiated") rat hepatocytes. This regimen is a useful model for experimental analysis of alterations in hepatocytes during carcinogenesis, and also as an assay for initiation by various carcinogens. Because of the decreasing availability of carcinogen-containing diets from commercial sources, we have developed alternative methods of 2-AAF administration to generate nodules in rats initiated with N-nitrosodiethylamine. This study compared the nodule-selecting and cancer-promoting efficacy of 2-AAF administered by the Solt-Farber procedure (0.02% in diet for 2 weeks) with 2-AAF administered by gavage, as a suspension in 1% aqueous carboxymethyl-cellulose (CMC). Three or 4 daily administrations of 2-AAF by gavage (20 mg/kg/day) followed by PH on day 4 were equivalent to the dietary regimen in generating early resistant nodules, late persistent nodules and hepatocellular carcinomas. These regimens were similar to the dietary regimen of 2-AAF in inhibiting virtually all normal hepatocyte proliferation. These regimens permit control over the duration and level of 2-AAF exposure and the resulting size of selected nodules.

Carcinogen-induced mdr overexpression is associated with xenobiotic resistance in rat preneoplastic liver nodules and hepatocellular carcinomas

We have previously reported the isolation of a human breast cancer cell line resistant to doxorubicin (adriamycin; AdrR MCF-7 cells) that has also developed the phenotype of multidrug resistance (MDR). MDR in this cell line is associated with increased expression of mdr (P glycoprotein) gene sequences. The development of MDR in AdrR MCF-7 cells is also associated with changes in the expression of several phase I and phase II drug-detoxifying enzymes. These changes are remarkably similar to those associated with development of xenobiotic resistance in rat hyperplastic liver nodules, a well-studied model system of chemical carcinogenesis. Using an mdr-encoded cDNA sequence isolated from AdrR MCF-7 cells, we have examined the expression of mdr sequences in rat livers under a variety of experimental conditions. The expression of mdr increased 3-fold in regenerating liver. It was also elevated (3- to 12-fold) in several different samples of rat hyperplastic nodules and in four of five hepatomas that developed in this system. This suggests that overexpression of mdr, a gene previously associated with resistance to antineoplastic agents, may also be involved in the development of resistance to xenobiotics in rat hyperplastic nodules. In addition, although the acute administration of 2-acetylaminofluorene induced an 8-fold increase in hepatic mdr-encoded RNA, performance of a partial hepatectomy either before or after administration of 2-acetylaminofluorene resulted in a greater than 80-fold increase in mdr gene expression over that in normal untreated livers. This represents an important in vivo model system in which to study the acute regulation of this drug resistance gene.

Mitogenic activity in platelet-poor plasma from rats with persistent liver nodules or liver cancer

Platelet-poor plasma (PPP) from F-344 rats with chemically-induced preneoplastic liver nodules or hepatocellular carcinoma stimulated S-phase DNA synthesis in monolayer cultures of normal rat hepatocytes. Similar mitogenic activity was detected in PPP 6 hrs to 1 week after partial hepatectomy (PH) or after necrotizing doses of CCl4 or diethylnitrosamine (DENA). Very little activity was found in PPP4 from control rats. The mitogenic activity in PPP from animals with nodules was non-dialyzable (greater than 14 kd) and bound to a heparin-sepharose affinity column. None of the mitogenic PPPs competed with [125I] epidermal growth factor (EGF) for binding sites on A431 cells or normal rat hepatocytes. These studies indicate that persistent proliferation of preneoplastic and neoplastic hepatocytes is associated with increased circulating levels of mitogenic hepatocyte growth factor.

Progression in hepatocarcinogenesis: differences in growth and behavior of transplants of early and later hepatocyte nodules in the rat spleen

Hepatocyte nodules were induced in Fischer 344 rats using the resistant hepatocyte model. Nodules harvested at 5, 24, or 25 weeks after initiation were isolated, diced, and transplanted into the spleen of normal rats and observed for periods up to 104 weeks. In the first experiment, 50% of the animals developed hepatocellular carcinoma, some with invasion and metastasis, by 70 to 104 weeks. In the second experiment, transplants of 5-week nodules grew very slowly and diffusely in the spleen, as did normal liver, but retained at least some of their phenotypic properties. In contrast, transplants of 25-week nodules grew into nodules up to 2.5 cm in diameter by 70 weeks. Two of the larger nodules had smaller nodules within resembling trabecular carcinoma. Transplants from the liver surrounding the 25-week nodules did not grow and produced no nodules by 70 weeks after transplantation. The implications of these observations in the study of progression in hepatocarcinogenesis are discussed briefly.

Initiation of carcinogenesis by a dietary deficiency of choline in the absence of added carcinogens

The feeding for 10 or 11 weeks of young male Fischer-344 rats, a diet devoid of choline and low in methionine, leads to the appearance of gamma-glutamyltransferase-positive foci of altered hepatocytes in the liver and to the induction of initiated resistant hepatocytes. The latter are known to contain the primary precursor cells for the ultimate development of hepatocellular carcinoma. This initiation of carcinogenesis with the choline-devoid diet is prevented by added choline. These observations indicate that a dietary deficiency may, by itself, without known contaminating or added carcinogens, initiate the carcinogenic process.

Patterns of ligand binding to normal, regenerating, preneoplastic, and neoplastic rat hepatocytes

The binding of epidermal growth factor, asialoorosomucoid, and apoprotein E-rich lipoproteins to isolated hepatocytes was investigated at various time intervals during the step-by-step development of liver cancer in rats. The degree of binding of the three ligands showed a progressive reduction in early persistent and late persistent putative preneoplastic hepatocyte nodules. This was further decreased in hepatocytes isolated from unequivocal hepatocellular carcinomas. Regenerating liver hepatocytes bound lesser amounts of epidermal growth factor and asialoorosomucoid than did hepatocytes from control resting liver but increased amounts of apoprotein E-rich lipoproteins. The progressive decrease in ligand binding during the precancerous phase of hepatocarcinogenesis, the nodule-to-cancer sequence, may render nodules less responsive to the influences of their external environments.

Influences of diethylnitrosamine on longevity of surrounding hepatocytes and progression of transplanted persistent nodules during phenobarbital promotion of hepatocarcinogenesis

The possibility that phenobarbital (PB) selectively promotes liver nodule development by decreasing survival of surrounding hepatocytes previously exposed to diethylnitrosamine (DENA) was evaluated. Livers of F-344 rats were labelled with [3H-methyl]-thymidine (3H-TdR) during developmental or regenerative growth. Neonatal rats given 3H-TdR between days 3 and 12 were subjected at 12 weeks of age to partial hepatectomy (PH) followed 24 hr later by DENA (10 mg/kg) or saline. Subsequent administration of PB (0.1% in drinking water) for 28 weeks reduced total liver label to 46 +/- 10% (saline group) or 40 +/- 4% (DENA group). Adult male rats initiated with DENA (200 mg/kg) and later labelled with 3H-TdR after PH also lost total liver label during 28 weeks' promotion with PB (0.05% in water) at rates similar to those exhibited by noninitiated rats given PB, and by DENA-treated or control rats not given PB. Large persistent (12 weeks) liver nodules generated by DENA in the Solt-Farber model were transplanted as small fragments into the spleens of syngeneic rats previously given 0, 100 or 200 mg/kg of DENA. Subsequent exposure to PB (0.05% in drinking water for 40 weeks) or Aroclor 1254 (6 X 300 mg/kg per month) promoted nodule and cancer development only in livers of DENA-initiated recipients. Surviving transplanted nodules remained as small microscopic clusters even after 40 weeks of promotion. However, PB increased transplant survival (50% vs. 21% in controls) whereas Aroclor reduced it to 8%. These findings indicate that promotion of liver nodules by PB occurs without enhanced mortality of surrounding hepatocytes previously damaged by DENA. They further suggest that promoters such as PB and PCBs do not directly influence the progression of established persistent nodules.

Modulation of calcium by the carcinogenic process in the liver induced by a choline-deficient diet

A diet devoid of choline and low in methionine (CD), without any added carcinogen, has been shown to induce 100% preneoplastic nodules and more than 50% cancer in the rat liver. Attempts to understand the mechanism by which a CD diet induces liver cell cancer revealed that like chemical carcinogens, a CD diet also appears to cause alterations in DNA, perhaps mediated by free radicals. Indeed, a CD diet induces nuclear lipid peroxidation prior to the changes in DNA. The CD diet induced DNA alterations coupled with continuing liver cell proliferation may account for the induction of initiated hepatocytes by the CD diet. To gain insight into the nature of free radicals generated by the CD diet, experiments were designed to determine whether agents that modulate free radical effects influence the CD diet induced changes in the liver. We investigated the effect of Ca2+ in the modulation of CD diet induced alterations in the liver. The results show that extra Ca2+ when added to the CD diet prevented some of the early changes due to choline deficiency, such as nuclear lipid peroxidation and DNA damage, but had little or no effect on the triglyceride accumulation in the liver. Also, the same CD diet with extra Ca2+, when used as a promoter after initiation by diethylnitrosamine, decreased the number and size of early putative preneoplastic foci and nodules.

Identification of a characteristic cytosolic polypeptide of rat preneoplastic hepatocyte nodules as placental glutathione S-transferase

Evidence is presented that a distinctive type of glutathione-S-transferase (GSHTase-P), and a cytosolic polypeptide of Mr 52,000 (P-52), each appearing in greatly increased amounts in hepatocyte nodules during liver carcinogenesis in the rat, are so far indistinguishable. The probable identity of the two polypeptides was established with the use of SDS polyacrylamide gel electrophoresis and Western blot techniques with purified GSHTase-P and P-52 and their respective antibodies and by comparison of the sequence of the first 26 N-terminal amino acids. Since the enzyme and the polypeptide are each considered to be the best available early markers for hepatocyte nodules, as putative precancerous lesions, their probable identity makes them attractive cellular components for in depth studies on their transcriptional and translational regulation and their use in new approaches to the sequential analysis of liver carcinogenesis.

Pathogenesis of experimental liver cancer: comparison with humans

The multi-step nature of cancer development in the liver with chemical carcinogens is very well developed in the rat and is highly probable in humans and in mice. Since this step-by-step analysis of hepatocarcinogenesis is much more advanced in the rat than in the other two species, the pathogenesis in the rat will be briefly reviewed. Initiation steps I and II, promotion steps III and IV and progression steps V and VI will be highlighted together with further unknown steps in progression to cancer. The focal proliferation of hepatocytes, as represented by hepatocyte nodules, appears to play a central and critical role as a precursor for cancer. In the rat, nodules have a very characteristic phenotype biologically, histologically and biochemically. Some of the options available to nodules and their significance will be discussed. The possible analogy between rat on the one hand and mice and humans on the other will be presented. A critical assessment of the relevance of pathological changes seen in rats and mice to the possible carcinogenicity of agents in humans will be discussed. The urgent need for more step-by-step studies of hepatocarcinogenesis in mice is very obvious and must be stressed.

Allogeneic hepatocyte transplantation in the rat spleen under cyclosporine immunosuppression

An innovative approach for stimulating the rapid growth of allogeneic hepatocytes implanted into splenic tissue with maintenance of the structural integrity is described. Single cell suspensions of hepatocytes from normal male ACI-strain rats (RTIa) were injected (2 X 10(6) cells) into the spleen of allogeneic male Fischer (RTI1) recipient rats. A 70% partial hepatectomy (PH) was performed at the same time as hepatocyte transplantation. Animals were treated for 4 days prior to, and 1 day after, transplantation with a feeding regimen containing 0.05% 2-acetylaminofluorene (AAF) to inhibit regeneration of the residual host liver. Animals received cyclosporine (CsA) 3 mg/kg/day s.c. posttransplantation. Histological examination of a standard longitudinal section of the recipient spleen two days posttransplant revealed an approximately 0.54-mm2 area replaced by hepatocytes. By 7 days this had increased to 0.97 +/- .15 mm2. Without CsA administration, hepatocytes were undetected at 7 days. Both PH and AAF treatment were necessary for successful colonization and sustained proliferation. Withdrawal of CsA treatment at 10 days after transplantation resulted in rapid rejection of established hepatocytes. This study demonstrates that rapid colonization of the rat spleen with allogeneic hepatocytes can be achieved, and that the viability and structural integrity of these transplanted cells can be maintained for at least 14 days using cyclosporine immunosuppression.

A choline-devoid diet, carcinogenic in the rat, induces DNA damage and repair

A diet deficient in choline when fed to rats for as few as 3 days resulted in liver DNA damage. The damage could be monitored as alkali-labile lesions using alkaline-sucrose gradients as well as alkaline elution technique. The DNA damage disappeared when the choline-deficient diet was replaced by a choline-supplemented diet suggesting the alkali-labile lesions were repairable. The DNA damage was detected at a time when no liver cell death was detectable. The induction of DNA damage in a proliferating liver by the choline-deficient diet may be an early important event leading to initiation of liver carcinogenesis.

Sequential alterations in growth control and cell dynamics of rat hepatocytes in early precancerous steps in hepatocarcinogenesis

This set of experiments is the second of a series designed to explore alterations in cell dynamics and growth control of new populations of hepatocytes that appear to play a role in the carcinogenic process induced in the liver by chemical carcinogens. This is part of an ongoing study of the biochemical and molecular basis for cancer development. A rat model for hepatocarcinogenesis, the resistant hepatocyte model, was chosen with its synchrony of several steps in the process. Carcinogenesis was initiated by the administration of a single necrogenic dose of diethylnitrosamine. Resistant hepatocytes so induced were stimulated to proliferate rapidly to form nodules by a mitogenic stimulus in the presence of a brief exposure to dietary 2-acetylaminofluorene sufficient to inhibit the proliferation of the majority of uninitiated hepatocytes, the nonresistant population. A small subset of these hepatocyte nodules, the persistent nodules, was examined at 2, 4, and 6 mo postinitiation. Duration of phases of the cell cycle, growth fraction, doubling time, cell death, and cell loss and the responses and subsequent recovery after the application of a strong mitogenic stimulus, partial hepatectomy, were measured. The first precancerous hepatocyte nodule, at 2 mo, showed a "normal" duration of phases of the cell cycle. The growth fractions were about 4,4, and 8% at 2, 4, and 6 mo, respectively, as compared to 0.4% in the surrounding hepatocytes. Accompanying the increased growth fractions were considerable levels of cell loss, measuring about 3% at 2 mo and 7% at 6 mo. At 6 mo, the hepatocyte nodule population, unlike the hepatocytes in the surrounding liver, shows a failure to return to its base-line level after stimulation of cell proliferation by partial hepatectomy. The results of this study have identified two new steps in the early precancerous phase of hepatocarcinogenesis relating to alterations in the control of cell proliferation and are consistent with the hypothesis that new and evolving cell populations may play an important role in the step-by-step carcinogenic process. These new populations appear to acquire alterations in growth control in a seriatim fashion, with retention of some "normal" properties.

The pattern of metabolism of 2-acetylaminofluorene in carcinogen-induced hepatocyte nodules in comparison to normal liver

This study was designed to test further the hypothesis that the special biochemical pattern seen in hepatocyte nodules during liver carcinogenesis could be of fundamental importance in their selective metabolism of one carcinogenic xenobiotic, 2-AAF, as related to their resistance to xenobiotics. Nodules of a certain stage were induced using the resistant hepatocyte model. The metabolism of a single small dose of 2-AAF in hepatocyte nodules in comparison to normal liver was studied at different time intervals up to 30 h. The levels of free 2-AAF in nodules and in normal liver were approximately the same over the whole time period. However, the nodules showed a large decrease in the binding of 2-AAF to DNA, RNA and proteins as well as in the metabolic conversion to hydroxylated forms, both free and conjugated with glucuronic acid. The patterns of metabolic conversion to metabolites and of conjugation of the metabolites are in harmony with the known biochemical patterns in nodules, a decrease in phase I components involved in the metabolism of carcinogens and other xenobiotics and an increase in most phase II components involved in conjugation and detoxification.

Influences of different polychlorinated biphenyls on cytocidal, mitoinhibitory, and nodule-selecting activities of N-2-fluorenylacetamide in rat liver

The influences of different polychlorinated biphenyl (PCB) isomers and congeners on distinct hepatotoxic responses to the hepatocarcinogen N-2-fluorenylacetamide [(2-FAA) CAS: 53-96-3] were examined in F344 rats. Cytocidal toxicity of 2-FAA (25-400 microM), determined by lactate dehydrogenase release during 20 hours in primary monolayer cultures of isolated rat hepatocytes, was reduced by in vivo pretreatment with either phenobarbitone [(PB) CAS: 50-06-6] or 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP), a PB-type PCB inducer. However, cytocidal toxicity of 2-FAA was substantially potentiated by either 3-methylcholanthrene [(MCA) CAS: 56-49-5] or 3,3',4,4'-tetrachlorobiphenyl [(TCBP) CAS: 32598-13-3], an MCA-type PCB. In the same cell culture assays, all four pretreatments similarly reduced cytocidal toxicity of N-hydroxy-N-2-fluorenylacetamide (0.32-32 microM; CAS: 53-95-2). By comparison, pretreatments with either the PB-type or MCA-type PCB's (50-200 mumol/kg) diminished mitoinhibitory toxicity of 2-FAA in vivo, as measured by hepatic regenerative growth and hepatocyte labeling indices 7 days after partial hepatectomy (PH) in rats given 3 consecutive daily doses of 2-FAA (20/mg/kg/day) before PH. This regimen of 2-FAA and PH promoted rapid selective growth of gamma-glutamyltranspeptidase-positive (gamma-GT+) nodules at 2 and 4 weeks after PH in rats previously given an initiating hepatocarcinogen, diethylnitrosamine [(DENA) CAS: 55-18-5]. However, various PCB's, including 2,2',4,4',5,5'-HCBP, 3,3',4,4'-TCBP, 2,2',4,4'-TCBP, 2,2',5,5'-TCBP, and the commercial mixture Aroclor 1254, each given as a single dose of 50 mumol/kg by gavage 10 days after DENA and 7 days before 2-FAA, all reduced the size of 2-FAA-selected gamma-GT+ nodules during the 4-week period after PH. These results indicate that, in spite of predictable inducer-specific opposite influences of different types of PCB's on cytocidal toxicity of 2-FAA, all PCB's similarly reduce nodule selection by 2-FAA in initiated livers. Reduced growth of 2-FAA-selected nodules correlated with the consistent ability of all PCB's to enhance regeneration of liver mass after 2-FAA and PH.

Ethionine in the analysis of the possible separate roles of methionine and choline deficiencies in carcinogenesis

The importance of ethionine, the ethyl analogue of methionine, as a metabolic probe to study the possible roles of methionine and choline in liver carcinogenesis has been briefly reviewed. Ethionine-induced liver carcinogenesis is similar in many aspects, including initiation, promotion, and progression, to carcinogenesis with other agents. However, the special role of methionine in preventing virtually all metabolic and pathologic effects of ethionine, including liver cancer, places ethionine in a special position. On the basis of these observations and our current knowledge about choline deficiency in the genesis of liver cancer, we proposed that choline and methionine play separate but overlapping roles in the initiation and promotion of liver carcinogenesis.

1,2-Dibromoethane initiation of hepatic nodules in Sprague-Dawley rats selected with Solt-Farber system

Initiating effects of the fumigant 1,2-dibromoethane (DBE) for liver were examined using the Solt-Farber selection system. Male Sprague Dawley rats (230-260 g) were given one oral dose of DBE (75 mg/kg), a two-thirds partial hepatectomy 4 h later, five oral doses of 2-acetylamidofluorene (AAF) (25 mg/kg) on days 17-21, CCl4 (2 ml/kg) on day 22, a booster dose of AAF (10 mg/kg) on day 32, and were sacrificed on day 82. Nodules and appreciable gamma-glutamyl transpeptidase foci were found in the livers of four of six animals given DBE, but not in any animals of the control group.

Initiation and selection of resistant hepatocyte nodules in rats given the pyrrolizidine alkaloids lasiocarpine and senecionine

The biological mechanisms by which pyrrolizidine alkaloids contribute to initiation and nodule selection (promotion) steps in hepatic carcinogenesis were studied in male Fischer 344 rats. Lasiocarpine at single or double dosages (up to 80 mumol/kg) delayed hepatic regeneration for at least 8 weeks after partial hepatectomy (PH). This regimen of lasiocarpine and PH had a strong selective influence on the growth of gamma-glutamyltranspeptidase (gamma-GT)-positive hepatocyte nodules in rats previously initiated with diethylnitrosamine. However, both lasiocarpine (up to 80 mumol/kg) and senecionine (up to 160 mumol/kg) were inactive as initiators of gamma-GT-positive nodules in rats exposed to a similar selection regimen consisting of 2-acetylaminofluorene and PH. When lasiocarpine or senecionine was given 12 h after PH, very few nodules were initiated. Lasiocarpine pretreatments reduced the initiating activity of diethylnitrosamine and N-nitrosomethylurea in rats subsequently selected with 2-acetylaminofluorene and PH. Resistant nodules selected with lasiocarpine had the typical resistant nodule phenotype (positive for gamma-GT and epoxide hydrolase) and also lacked pyrrolizidine alkaloid-induced megalocytosis. Lasiocarpine treatment also resulted in small regenerative nodular proliferations of hepatocytes that were distinct from resistant nodules because they were negative for gamma-GT and epoxide hydrolase and unrelated to diethylnitrosamine pretreatments. These studies suggest that the hepatocarcinogenicity of pyrrolizidine alkaloids can be better explained by their strong selection (promotion) influence on initiated hepatocytes, rather than by their very weak initiating activity.